This invention relates to copying apparatuses, and more particularly to microfilm copying apparatuses
A microfilm copying apparatus by which a content is copied from a master film to an unexposed film involves a roll-formed master film (a silver salt film) and a roll-formed unexposed film (a silver salt film or a diazo film), and these films are brought into close contact with each other on the circumference of a printing drum to expose the unexposed film at a predetermined position thereon.
Such a microfilm copying apparatus comprises, as shown in FIG. 9 with a front view thereof a master unwind shaft 1 for unwinding a master film A; master brake rollers 2 for providing a braking force in a direction opposite to that of flow of the master film A; a printing drum 3 for providing contact and exposure; an exposing unit 4 for exposing a film; master pull rollers (drive and pull unit) 5 for pulling the master film A; a master rewind shaft 6 for rewinding the master film A; a diazo unwind shaft 7 for unwinding the unexposed film (e.g., a diazo film) B; a developing unit 8 for developing the exposed diazo film B; a diazo pull roller 9 for pulling the diazo film B; and a diazo rewind shaft 10 for rewinding the diazo film B.
The exposing unit 4 that serves to copy a content of the master film A to the diazo film B is located at a predetermined position on the circumference of the printing drum 3 so that light will be injected onto the film surface vertically.
The master film A is externally wound on the main surface of the diazo film B that is wound on the circumference of the printing drum 3.
In the above-described microfilm copying apparatus is constructed a tension is produced on the master film A wound on the circumference of the printing drum 3 by both a braking force derived mainly from the master brake rollers 2 and a drive force derived mainly from the master pull rollers 5. That is, the master film A is acted upon by a force directed toward the axis of the printing drum 3, or a centripetal force, and this centripetal force presses the diazo film B wound internally on the circumference of the printing drum 3, thereby improving the degree of contact between the master film A and the diazo film B.
However, the degree of contact depends on the tension defined by the drag and pulling forces of the master film A, and in the system of producing a tension by the forces of the master brake rollers 2 and the master pull rollers 5 such as above, it is difficult to maintain the proper tension for the roll-formed master film A as its diameter varies from a maximum to a minimum (the diameter of the unwind roll will reduce by about 1/5 the original diameter and that of the rewind roll will increase by about 5 times the original). In such a case, a measure to reducing tensile variations by increasing the tension of the master film A is not a satisfactory solution in that it causes deformation of the master film A including elongation and damage and thus causes nonuniform contact between the master film A and the diazo film B with resultant blurs in a copied image.
Further, the speed of the master film A is, on the one hand, determined by the difference between the drag and pulling forces of the master film A. On the other hand, the speed of the diazo film B is determined by a portion of difference between the drag force of the unexposed film B (a braking force of the diazo film B and a frictional force between the diazo film B and the printing drum 3 and the pulling force (a drive force derived mainly from the diazo pull roller 9) and a portion depending on the speed of the master film A taking into account the frictional force defining the degree of contact between both films A and B. In addition, the printing drum 3 is an idle drum, which is also subject to a drive by the frictional force between the diazo film B and the printing drum 3 caused due to the tension of each of the films A and B. Therefore, in such a system the stability of the speed of the master film A achieved by accurately controlling the speed of the master pull rollers 5 would not contribute to eliminating the fluctuations of the degree of contact, thereby causing differences in speed between both films and producing shifts in the copied image. This imposes a restriction on the utilization of the apparatus; i.e., a copying can be made only at lower speeds (6 m/min or lower) where speed differences are insignificant.
Still further, the developing unit 8 for developing the diazo film B cannot provide sufficient circulation of the hot blast that contains a mixture of an ammonia gas and steam due to a large difference in surface area between the upper and lower vent holes. As a result, there exists a temperature difference of about 35.degree. C. between both vent holes (about 115.degree. C. at the upper vent hole and about 80.degree. C. at the lower vent hole), thereby causing variations in tone at the initial, intermediate, and end phase during the development of the diazo film B.
This invention has been made to overcome the above problems.
An object of this invention, therefore, is to provide a microfilm copying machine for copying a content from a master film to an unexposed film capable of preventing copied images from being blurred, shifted, or subjected to tone variations.